A contact image sensor (CIS) is a device which detects optical signals and converts them into readable electric signals. It is applied in image processing devices requiring electro-optical components for reading texts and pictures data such as scanners and copiers.
As shown in FIG. 1, after a document (not shown) is fed onto a transparent glass 73 through a transmission axis 71, a light source 79 will illuminate the document. The reflected light passes through a lenticular lens 75, which proportionally projects the reflected image onto a CIS chip 77 of a CIS module 70 in an identical ratio. The CIS chip 77 then detects the light signals to output electric signals in order.
The CIS module 70 is formed by series connecting several CIS chips 77 to match the size of a document to be read. That is, the larger the size of a document to be read, the more the CIS chips 77 used on the CIS module 70. The total length of the series-connected chips should be larger than the width of the document.
As shown in FIG. 2, a conventional image sensing module comprises an optical sensor set 81, a switch array 83, a shift array 0.85 and an amplifying circuit 87. In this embodiment, the resolution of the optical sensor set 81 is 600 dpi (dots per inch). In other words, the optical sensor set 81 includes 600 optical sensors D1˜D600. The optical sensors D1˜D600 can be photodiodes or phototransistors. The switch array 83 is composed of 600 switches, which are connected to the output terminals of the optical sensors one by one. The switch array 83 is used for controlling transmission of charge signals accumulated by the optical sensors. That is, after any optical sensor accumulates an enough charge energy, the corresponding switch will be on. The shift array 85 is composed of shift registers of the same number as the switches. The shift array 85 is used to respond to the input of a timing signal CK to control in order the activation of corresponding switches so that signals in the optical sensors can be outputted in order according to pixel positions.
The amplifying circuit 87 is connected to the output terminal of the shift array 85. The amplifying circuit 87 comprises an operation amplifier (op_amp), two switches SW1 and SW2, and a capacitor CL, and is used to amplify the output signal of the shift-array 85 for subsequent read and processing. The scanned image signals can thus be obtained.
For an existent CIS, the readout of signals is accomplished by accumulating sensed optical signals in optical sensors in charge form. The accumulated charges are then outputted one by one when outputting pixels. For the architecture shown in FIG. 2, the produced timing diagram is shown in FIG. 3. Through control of the on time of the switches SW1 and SW2, each charge signal outputted by the shift array 85 can be outputted and amplified in order, and a reset time (controlled by the switch SW2) is provided between adjacent charge signals. After the reset is finished, the next charge signal of the shift array 85 is then outputted.
When the signal of an existent CIS is read, charge signals obtained by detecting light using optical sensors are outputted one by one. The higher the resolution, the more the optical sensors used. Therefore, in order to completely output the charge signals of all the optical sensors, the required time is elongated. For the architecture of an existent image sensing module, if the speed of signal output is to be increased, one can raise the clock of the shift array to fast switch the conduction time of each optical sensor for outputting the charge signal.
Although shortening the conduction time of each optical sensor for outputting the charge signal can speed up the signal output of the image sensing module, there is still a bottleneck to completely output the charge signals accumulated by the optical sensors in such a short time due to the charging/recharging time of capacitor, which is further limited by the area size of the optical sensors and the detection capability in an unit time.
Accordingly, the present invention aims to provide an image sensing module capable of fast transferring signal and a method thereof to solve the above problems in the prior art.